Re: [PATCH] RTIC: selinux: ARM64: Move selinux_state to a separate page

[Ard Biesheuvel <ardb@kernel.org>]

On Wed, 17 Feb 2021 at 10:42, Will Deacon <will@kernel.org> wrote:
>
> [Please include arm64 and kvm folks for threads involving the stage-2 MMU]
>
> On Tue, Feb 16, 2021 at 03:47:52PM +0530, Preeti Nagar wrote:
> > The changes introduce a new security feature, RunTime Integrity Check
> > (RTIC), designed to protect Linux Kernel at runtime. The motivation
> > behind these changes is:
> > 1. The system protection offered by Security Enhancements(SE) for
> > Android relies on the assumption of kernel integrity. If the kernel
> > itself is compromised (by a perhaps as yet unknown future vulnerability),
> > SE for Android security mechanisms could potentially be disabled and
> > rendered ineffective.
> > 2. Qualcomm Snapdragon devices use Secure Boot, which adds cryptographic
> > checks to each stage of the boot-up process, to assert the authenticity
> > of all secure software images that the device executes.  However, due to
> > various vulnerabilities in SW modules, the integrity of the system can be
> > compromised at any time after device boot-up, leading to un-authorized
> > SW executing.
> >
> > The feature's idea is to move some sensitive kernel structures to a
> > separate page and monitor further any unauthorized changes to these,
> > from higher Exception Levels using stage 2 MMU. Moving these to a
> > different page will help avoid getting page faults from un-related data.
> > The mechanism we have been working on removes the write permissions for
> > HLOS in the stage 2 page tables for the regions to be monitored, such
> > that any modification attempts to these will lead to faults being
> > generated and handled by handlers. If the protected assets are moved to
> > a separate page, faults will be generated corresponding to change attempts
> > to these assets only. If not moved to a separate page, write attempts to
> > un-related data present on the monitored pages will also be generated.
> >
> > Using this feature, some sensitive variables of the kernel which are
> > initialized after init or are updated rarely can also be protected from
> > simple overwrites and attacks trying to modify these.
>
> Although I really like the idea of using stage-2 to protect the kernel, I
> think the approach you outline here is deeply flawed. Identifying "sensitive
> variables" of the kernel to protect is subjective and doesn't scale.
> Furthermore, the triaging of what constitues a valid access is notably
> absent from your description and is assumedly implemented in an opaque blob
> at EL2.
>
> I think a better approach would be along the lines of:
>
>   1. Introduce the protection at stage-1 (like we already have for mapping
>      e.g. the kernel text R/O)
>
>   2. Implement the handlers in the kernel, so the heuristics are clear.
>
>   3. Extend this to involve KVM, so that the host can manage its own
>      stage-2 to firm-up the stage-1 protections.
>

Agree here. Making an arbitrary set of data structures r/o behind the
OS's back doesn't seem like an easy thing to maintain or reason about,
especially if this r/o-ness is only enforced on a tiny subset of
devices. If something needs to be writable only at boot, we have
__ro_after_init, and having hypervisor assisted enforcement of /that/
might be a worthwhile thing to consider, including perhaps ways to do
controlled patching of this region at runtime.

> I also think we should avoid tying this to specific data structures.
> Rather, we should introduce a mechanism to make arbitrary data read-only.
>
> I've CC'd Ard and Marc, as I think they've both been thinking about this
> sort of thing recently as well.
>
> Will